1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an automotive engine. More particularly, the invention relates to a device for purifying exhaust gas of an engine in which a permeability of a particulate filter mounted on a diesel engine is evaluated by calculating the apparent passage area based upon a difference in the pressure between the upstream and the downstream sides of the particulate filter and upon other engine data, an alarm is given to a driver based on the results of evaluation, and the particulate filter is properly regenerated.
2. Description of the Related Art
In order to prevent air pollution, there has heretofore been widely used a particulate filter for trapping particulate matter emitted from automotive engines and, particularly, from diesel engines. Particulate filters of a variety of types have been proposed. Among them, the particulate filter of the type which automatically burns the particulate matter which it has trapped, so as to be regenerated without the need of dismantling it, is simple in constitution and is very desirable. When the particulate matter is deposited to an excess degree, the particulate filter of this type loses the permeability in the exhaust gas passage, causing an undesired drop in the engine output and an increased emission of smoke. It is therefore necessary to properly execute the regeneration processing while monitoring the permeability of the particulate filter at all times. If the particulate filter breaks, particulate matter is released to the open air. It is therefore necessary to detect abnormal permeability of the particulate filter and to turn an alarm lamp on to let the driver know this fact to allow him to have the particulate filter repaired.
As a method of monitoring the permeability of the particulate filter, there has been known a method of monitoring the differential pressure between the upstream and the downstream sides of the particulate filter by using a differential pressure sensor (pre-exhaust gas sensor and post-exhaust gas sensor). As a method featuring a higher precision, Japanese Unexamined Patent Publication (Kokai) No. 60-47937 proposes a method of finding an apparent passage area from such data as flow rate of the air taken in by the engine, exhaust gas temperature and exhaust gas pressure relying upon the Bernoulli""s flow formula. The method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-47937 features high precision compared to that of the method which simply monitors the differential pressure. When it is attempted to detect the exhaust gas pressure and the differential pressure between the upstream and the downstream of the particulate filter in an averaged manner, however, an averaging error occurs due to pulsation in the exhaust pipe resulting in a decrease in the precision.
FIG. 17 is a diagram illustrating an averaging error that occurs when it is attempted to find an average value of the pressure signals in the presence of the pressure pulsation. There exists a secondary functional relationship between the exhaust gas flow rate (G) and the differential pressure (xcex94) between the upstream and the downstream of the particulate filter. When there exists pulsation, the average values thereof deviate from a static secondary functional relationship to develop an average error. That is, even though it is attempted to find a differential pressure (xcex94) between the upstream and the downstream sides of the particulate filter corresponding to the average value of the exhaust gas flow rate (G), the value that is obtained tends to become large and involve an error. Even though it is attempted to effect the on-board diagnosis (OBD) function, therefore, it is not possible to detect the values when the apparent passage area is abnormally large or it is not possible to change, while maintaining a sufficient degree of reliability, the method of burning the particulate matter and regenerating the particulate filter in compliance with a decrease in the apparent passage area.
It is an object of the present invention to provide a device, for purifying exhaust gas of engines, which is capable of finding an apparent passage area of the particulate filter while maintaining a sufficiently high precision. The invention further provides a device for purifying exhaust gas of engines, which is capable of property regenerating the particulate filter. The invention further provides a device for purifying exhaust gas of engines capable of properly detecting the defective state such as breakage of the particulate filter to request the driver to have it repaired.
According to a first aspect of the present invention, the permeability of the particulate filter that traps particulate matter emitted from the automotive engine, is evaluated, i.e., the apparent passage area is calculated based upon the differential pressure between the upstream and the downstream of the particulate filter and upon a plurality of engine data, and alarm is produced when it is judged that the apparent passage area lies under predetermined conditions, making it possible to find the apparent passage area maintaining a sufficiently high precision and to precisely detect an abnormal increase in the passage area caused by a fault such as breakage of the particulate filter. Thus, the faulty state such as breakage of the particulate filter is properly detected and an alarm means is energized to request the driver to have the particulate filter repaired.
According to another aspect of the present invention, the permeability of the particulate filter that traps particulate matter emitted from the automotive engine is evaluated, i.e., the apparent passage area is calculated based upon the differential pressure between the upstream and the downstream side of the particulate filter and upon a plurality of engine data, and an alarm is produced when it is judged that the apparent passage area is larger than a predetermined value or is smaller than a predetermined value, making it possible to accomplish the same effect as the one described above. According to the present invention, it is also judged whether the rate of increase in the apparent passage area is larger than a predetermined rate, making it possible to properly detect the faulty state where the particulate filter is broken with the particulate matter being deposited on the particulate filter.
According to a further aspect of the present invention, the permeability of the particulate filter is evaluated, i.e., the apparent passage area is calculated based upon the differential pressure between the upstream and the downstream sides of the particulate filter and upon a plurality of engine data, an instruction for regenerating the particulate filter is output when it is judged based on the apparent passage area that the particulate filter having particulate matter deposited thereon to be regenerated, and a regeneration end instruction is output for ending the processing for regenerating the particulate filter when it is confirmed that the regeneration of the particulate filter has been finished, making it possible to reliably regenerate the particulate filter without the need of dismantling the particulate filter and without causing the particulate filter to be excessively heated, which is undesirable.
According to a still further aspect of the present invention, an average error caused by the pulsation of the exhaust gas pressure is corrected at the time of finding the apparent passage area by utilizing the differential pressure between the upstream and the downstream sides of the particulate filter, making it possible to improve the precision for calculating the apparent passage area, to find the apparent passage area maintaining a sufficient precision and to precisely detect an abnormal increase in the passage area caused by the breakage of the particulate filter. This prevents such an occurrence that the particulate matter is emitted to the open air for extended periods of time as a result of the breakage of the particulate filter.
According to a yet further aspect of the present invention, three or more data are selected as a plurality of engine data out of the flow rate of the air taken in by the automotive engine, exhaust gas temperature, temperature of the particulate filter, atmospheric pressure and flow rate of the fuel. Under the conditions where the flow rate of the air taken in by the automotive engine is small such as under an idling condition, the differential pressure between the upstream and the downstream sides of the particulate filter becomes very small, and the apparent passage area is not found sufficiently precisely. According to a further aspect of the present invention, therefore, a step for calculating the apparent passage area is discontinued when the flow rate of the air taken in by the engine is smaller than a predetermined value.
According to another aspect of the present invention, provision is made of operation condition change-over means for changing over the operation conditions of an automotive engine toward a direction in which the exhaust gas temperature is elevated and the particulate filter is heated in response to an instruction for regenerating the particulate filter, and, when it is confirmed that the processing for regenerating the particulate filter is finished, an instruction is given to the operation condition change-over means so as to return the operation conditions of the automotive engine back to the initial operation conditions. Usually, therefore, the automotive engine is operated under the conditions of good fuel efficiency maintaining the exhaust gas temperature low, making it possible to save the fuel.
According to a further aspect of the present invention, if the particulate filter is heated at a temperature at which the particulate matter oxidizes and burns quickly under a condition where it is so judged that a large amount of particulate matter has been deposited, then, the particulate matter may burn rapidly causing the particulate filter to be overheated. When a first instruction output means has detected an extreme drop in the apparent passage area, therefore, the operating conditions are so changed as to heat the particulate filter at a temperature at which the particulate matter deposited on the particulate filter mildly reacts, in order to mildly regenerate the particulate filter without causing the particulate filter to be overheated.
The present invention may be more fully understood from the description of preferred embodiments of the invention as set forth below together with the accompanying drawings.